(1) Field of the Invention
This invention relates to an apparatus and method for CMP (Chemical Mechanical Polishing) of a semiconductor substrate and more specifically to an improved CMP apparatus and method of CMP which results in improved polishing uniformity by varying the loading pressure of the substrate against the polishing pad.
(2) Description of Related Art
In the fabrication of semiconductor integrated circuits CMP (Chemical Mechanical Polishing) has been developed for providing smooth topographies on surfaces deposited on the semiconductor substrates. Rough topography results when metal conductor lines are formed over a substrate containing device circuitry. The metal conductor lines serve to interconnect discrete devices, and thus form integrated circuits. The metal conductor lines are further insulated from-the next interconnection level by thin layers of insulating material and holes formed through the insulating layers provide electrical access between successive conductive interconnection layers. In such wiring processes, it is desirable that the insulating layers have a smooth surface topography, since it is difficult to lithographically image and pattern layers applied to rough surfaces. CMP can, also, be used to remove different layers of material from the surface of a semiconductor substrate. For example, following via hole formation in an insulating material layer, a metallization layer is blanket deposited and then CMP is used to produce planar metal studs embedded in the insulating material layer.
Briefly, the CMP processes involve holding and rotating a thin, flat substrate of the semiconductor material against a wetted polishing surface under controlled chemical, pressure and temperature conditions. A chemical slurry containing a polishing agent, such as alumina or silica, is used as the abrasive material. Additionally, the chemical slurry contains selected chemicals which etch various surfaces of the substrate during processing. The combination of mechanical and chemical removal of material during polishing results in superior planarization of the polished surface.
An important challenge in CMP is to achieve uniform polishing across the semiconductor substrate and uniform thickness removal across the semiconductor substrate.
C. Y. Chang, S. M. Sez, in ULSI Technology, The McGrawHill Company, Inc., 1997, pp 439-442, discuss CMP techniques and CMP removal rates as a function of pressure and velocity of individual points on the wafer substrate. On p. 441, it is stated that the edge of a wafer substrate has a polishing rate equal to or higher than the center of the wafer. To compensate for this, the equipment uses a slightly convex curvature on the wafer carrier to exert a higher pressure toward the center of the wafer.
U.S. Pat. No. 5,643,053 entitled xe2x80x9cChemical Mechanical Polishing Apparatus With Improved Polishing Controlxe2x80x9d granted Jul. 1, 1997 to Norm Shendon describes a CMP apparatus and substrate carrier which controls the load force of the substrate against the polishing pad.
U.S. Pat. No. 5,643,061 entitled xe2x80x9cPneumatic Polishing Head For CMP Apparatusxe2x80x9d granted Jul. 1, 1997 to Paul David Jackson shows a CMP wafer carrier in which the carrier plate is thicker in the center than at the side wall in order to prevent the carrier plate from flexing when a pressure differential exists across the bottom plate.
U.S. Pat. No. 5,421,769 entitled xe2x80x9cApparatus For Planarizing Semiconductor Wafers, And A Polishing Pad For A Planarization Apparatusxe2x80x9d granted Jun. 6, 1995 to Laurence D. Schultz et al describes a CMP method in which a non-circular polishing pad is used to improve polishing uniformity.
U.S. Pat. No. 5,297,364 entitled xe2x80x9cPolishing Pad With Controlled Abrasion Ratexe2x80x9d granted Mar. 29, 1994 to Mark E. Tuttle shows a polishing pad with a face shaped to produce controlled non-uniform CMP rates.
U.S. Pat. No. 5,599,423 entitled xe2x80x9cApparatus And Method For Simulating And Optimizing A Chemical Mechanical Polishing Systemxe2x80x9d granted Feb. 4, 1997 to Norman W. Parker et al shows a CMP apparatus used to simulate CMP processes and study CMP variables, such as polishing uniformity.
The present invention is directed to a novel CMP apparatus and CMP method which achieve uniform polishing across the semiconductor substrate and uniform thickness removal across the semiconductor substrate.
It is a general object of the present invention to provide an improved and new apparatus and method for achieving uniform polishing and uniform thickness removal across a semiconductor substrate.
A more specific object of the present invention is to provide an improved CMP apparatus and method for achieving uniform polishing and uniform thickness removal across a semiconductor substrate, in which the pressure between the polishing pad and the semiconductor substrate is adjusted to produce uniform material removal.
Another object of the present invention is to provide an improved CMP apparatus method for achieving uniform polishing and uniform thickness removal across a semiconductor substrate, whereby the method of adjusting the local pressure between the polishing pad and the semiconductor substrate is simple and of low cost.
In accordance with the present invention, the above and other objectives are realized by a substrate carrier head for chemical mechanical polishing, comprising: a flexible carrier plate to hold the substrate against a polishing pad, said flexible carrier plate being connectable to a drive shaft to rotate with said drive shaft; and a plurality of nested concentric cylinders placed on said flexible carrier plate to exert pressure between said flexible carrier plate and said polishing pad.
In another embodiment of the present invention, the above and other objectives are realized by using a chemical mechanical polishing method for removal of material from the surface of a substrate, the uniformity of removal of material being improved through the steps of: providing a polishing pad affixed to a rotatable polishing platen; providing a rotatable and flexible carrier plate to hold said substrate against said polishing pad; providing a plurality of nested concentric cylinders placed on said flexible carrier plate to exert pressure between said flexible carrier plate and said polishing pad; dispensing a polishing slurry onto said rotatable polishing pad; providing a first means to rotate said flexible carrier plate; and providing a second means to rotate said polishing platen.